The present invention relates to fluid controllers of the type used to control the flow of fluid from the source of pressurized fluid to a fluid pressure operated device, such as a steering cylinder.
A typical fluid controller of the type to which the present invention relates includes a housing which defines various fluid ports, and further includes a fluid meter and valving, and an arrangement for imparting follow-up movement to the valving in response to flow through the fluid meter. The flow through the controller valving is directly proportional to the area of the main variable flow control orifice, which, in turn, is proportional to the rate at which the steering wheel is rotated. Furthermore, the area of the main variable flow control orifice has a know relationship to the displacement of the controller valving.
Therefore, it has long been an object of those skilled in the art to provide a steering system, including a fluid controller, in which the total flow through the steering system is substantially greater than the flow through the controller, but with the overall system flow being related to the flow through the controller in a known manner. See for example U.S. Pat. No. 4,052,929 in which the controller receives fluid from one pump and generates a pilot signal to control a pilot operated valve which receives fluid from a second pump. The total steering flow comprises the flow through the pilot operated valve plus the flow from the controller. Such a system is theoretically satisfactory, but the cost of such a system becomes nearly prohibitive because of the addition of the pilot operated valve and the second pump.
More recently, there has been an attempt to provide a flow to the steering cylinder which is greater than the flow through the fluid meter by having the full amount of desired steering flow enter the controller, with one portion flowing through the controller valving and fluid meter in the normal manner, and the remainder of the fluid flowing through a pressure regulating device and a bypass throttle. These two portions of fluid recombine within the controller and flow to the steering cylinder. See U.S. Pat. No. 4,566,272. It is possible that the performance of a controller made in accordance with U. S. Pat. No. 4,566,272 would be satisfactory, however, the addition of a pressure regulating valve within the controller, and the associated structure would still add substantially to the cost of the controller, and in many applications would require substantial redesign of at least the controller housing in order to accommodate the addition of such a valve.
U.S. Pat. No. 4,759,182, assigned to the assignee of the present invention, discloses a fluid controller in which the valving defines an amplification fluid path, including a variable amplification orifice in parallel with the main fluid path. Thus, the amplification fluid path provides an amplification ratio which can be made to vary, in any desired manner, as a function of valve displacement. It has been discovered, however, that when the amplification fluid path of U.S. Pat. No. 4,759,182 is applied to fluid controllers which are capable of a manual steering operation, any attempts to manually steer the vehicle are unsuccessful. More specifically, rotation of the steering wheel does not result in the buildup of pressurized fluid which is communicated to the steering cylinder to effect the manual steering operation.